This invention relates to short circuit protectors for electronic switches such as proximity switches. More particularly, this invention relates to a short circuit protector which is self-resettable upon each half-cycle of applied voltage.
Transistor circuits which function as a current limiting protective device for electronic switching devices such as target sensing solid state proximity switches are known. The protective circuit is connected in series with the load and the solid state switch and under normal conditions, operates in a low impedance state. Upon overload fault conditions, such as a short circuit across the load, the protective circuit switches to a high impedance state to limit the current flowing to the protected solid state switch. In one form, protective devices of the aforementioned type remain latched in the high impedance state until the power to the device is removed or dropped below some predetermined value. This resetting action normally requires that a workman determine and locate the fault condition and then perform some specific resetting operation.
Protective circuits of the aforementioned type which are automatically, or self-resettable, are also known. These circuits sense the short circuit on each half-cycle and repeatedly switch to the high impedance state when the amplitude of the sensed overcurrent reaches the trip point for the protective circuit. The repeated application of high currents to the protective circuit consumes a significant amount of power and can cause damage to the protective circuit due to the heat generated by the dissipation of this power.